Nonperturbative renormalization and O(a)-improvement of the nonsinglet vector current with N_f = 2 + 1 Wilson fermions and tree-level Symanzik improved gauge action

摘要

In calculating hadronic contributions to precision observables for tests of the Standard Model in lattice QCD, the electromagnetic current plays a central role. Using a Wilson action with O(a) improvement in QCD with Nf flavors, a counterterm must be added to the vector current in order for its on-shell matrix elements to be O(a) improved. In addition, the local vector current, which has support on one lattice site, must be renormalized. At O(a), the breaking of the SU(N_f) symmetry by the quark mass matrix leads to a mixing between the local currents of different quark flavors.We present a nonperturbative calculation of all the required improvement and renormalization constants needed for the local and the conserved electromagnetic current in QCD with N_f = 2 + 1 O(a)-improved Wilson fermions and tree-level Symanzik improved gauge action, with the exception of one coefficient, which we show to be order g60 in lattice perturbation theory. The method is based on the vector and axial Ward identities imposed at finite lattice spacing and in the chiral limit. We make use of lattice ensembles generated as part of the coordinated lattice simulations initiative.